Vehicles today use, for example, urea as a reductant in SCR (selective catalytic reduction) systems which comprise an SCR catalyst in which said reductant and NOx gas can react and be converted to nitrogen gas and water. Various types of reductants may be used in SCR systems. AdBlue is an example of a commonly used reductant.
One type of SCR system comprises a container which holds a reductant. The SCR system has also a pump arranged to draw said reductant from the container via a suction hose and to supply it via a pressurised hose to a dosing unit situated adjacent to an exhaust system of the vehicle, e.g. adjacent to an exhaust pipe of the exhaust system. The dosing unit is arranged to inject a necessary amount of reductant into the exhaust pipe upstream of the SCR catalyst according to operating routines which are stored in a control unit of the vehicle. To make it easier to regulate the pressure when there are small or no dosing amounts, the system may also have a return hose which runs back to the container from a pressure side of the system. According to this configuration it is possible to cool the dosing unit by means of the reductant which, during cooling, flows from the container via the pump and the dosing unit and back to the container. The dosing unit is thus provided with active cooling. The return flow from the dosing unit to the container may be substantially constant and is currently not controlled or regulated by means of purpose-made valves or such units.
The pump currently has a filter to filter the reductant before dosing via the dosing unit. This filter is provided to protect the dosing unit from becoming blocked by particles, e.g. soil particles, dirt etc. The filter may be a paper filter, but other kinds of filter may of course be used.
A service life of the filter is currently defined by suppliers of SCR systems as a predetermined cumulative distance travelled by the vehicle. It is recommended that an ordinary type of filter for the pump be replaced after 120,000 kilometers. A recommendation for commercial vehicles might be 80,000 km. In certain cases, these recommendations are implemented as a servicing operation for the respective vehicle, which means that service staff check on the occasion of a vehicle service how far it has travelled since the latest filter change, and replace said filter if the predetermined distance has been, or is close to being, reached.
There are various disadvantages of using a predetermined distance travelled as a measure of the degree of blockage of the filter.
One disadvantage is that applications where the SCR system doses more than average reductant during operation involve risk that the filter may become blocked before the vehicle has travelled the predetermined distance. This may cause impaired cooling of the dosing unit and consequent functional degradation and/or material destruction.
In cases of frequent use of applications where the vehicle's SCR system requires dosing of reductant despite the vehicle not being run and not accumulating distance travelled, the filter may likewise become blocked before the predetermined distance has been travelled.
In applications which dose less than an average, the predetermined distance may be too short, with the result that the filters actual service life is not fully utilised. This is negative from a cost perspective. It is also negative from a safety perspective through entailing increased risk of contamination of the reductant, and of the dosing system's actual components, on the occasion of filter replacement.
Another disadvantage of the state of the art is that using distance travelled as a basis for a replacement interval for the filter of the pump in the SCR system involves a quite considerable safety margin. This means that many filters are currently replaced too early, i.e. before replacement is really necessary. This is of course negative from a cost perspective.
It should also be noted that various applications of SCR systems are currently of a stationary nature, making it impossible to define a predetermined distance travelled as a key parameter for replacement intervals for the filter of the pump.
There is therefore a need to improve today's SCR systems by reducing or eliminating the above disadvantages.